My sister has a ten year old dog with arthritis starting to slow down the back legs. The neighbors have a young pup can’t be more than two. The two dogs love to play, jump and wrestle each other. They have a great time, but you can tell when the older dog has had enough. “Okay kid! I have had enough” Sometimes she runs away, but there is the occasional nip to say I have had enough.
That old doggie in the pic still has sharp eyes, though.
My dog is 14 years old and he just doesn’t even really play anymore. He’ll just semi-run behind the younger dogs trying to keep up and then hoarsely bark at them. Probably telling them to get off his lawn.